The invention relates generally to direct current machines and more particularly to direct current machine monitoring systems.
A degraded commutator of a direct current (DC) machine will show excessive sparking as brushes bounce over the rough surface and the conduction period for a particular segment ends prematurely or as a winding remains shorted for too long by a spark region which extends between segments. Ultimately a short circuit will develop, through an extended spark region over the commutator bars, between opposite-polarity brushes. This condition is known as “flashover”. Sparking can additionally be caused by factors such as worn down brushes, improper brush positioning, or load or supply problems, for example.
The principal mechanisms of brush wear are mechanical friction and sparking. Wear by friction is similar to friction wear in other mechanical systems. Frictional wear will be accelerated when the surface of the commutator is not smooth. Sparking will directly cause erosion by excessive local heating, field emission and other associated phenomena. Spark erosion also results in roughening of the commutator surface which further accelerates friction wear. A rough commutator then also causes increased sparking leading to accelerated brush wear and increasing commutator roughness so that the process feeds on itself.
The visual “sparking index” is a useful measure of commutation quality in DC motors. In the traditional method the number of sparking sites and their intensity on a single brush are visually observed and compared to a sample chart to get a rough estimate of severity. Using the chart, the sparking intensity is assigned a sparking index, which is a letter designation such as “A” for no sparking, “B” for minimal sparking, “C” for more sparking, and so on. Unfortunately, such visual observations are highly subjective, variable, and often times impractical due to viewing conditions or motor location.
On-line monitoring of commutation quality degradation as a measure of brush and/or commutator degradation or wear, for example, or as a precursor to flashover in DC machines can provide a significant advantage in steel mill, paper mill, and locomotive applications, for example, wherein visual inspection during operation is impractical. As described in Michael P. Treanor and Gerald B. Kliman, “INCIPIENT FAULT DETECTION IN LOCOMOTIVE DC TRACTION MOTORS,” Proceedings of the 49th Meeting of the Society for Machinery Failure Prevention Technology, Virginia Beach, Va., April 1995, pp. 221-230, machine condition monitoring for poor commutation can be achieved by frequency-domain analysis of machine current at the frequency of bar passing. The bar-passing frequency can be determined by multiplying the number of commutator bars by the speed (rotation frequency) of the motor. The magnitude of the peak at the bar-passing frequency increased by a factor of at least two when the commutation quality was degraded by incorrectly aligning the brushes. To provide an unambiguous determination of the bar passing frequency, the speed of the motor needs to be obtained with sufficient certainty and precision, which can be made difficult due to speed variations.